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Precambrian Basement Structure of the Diablo Platform and Its Influence through the Phanerozoic

Steve Grimes¹

¹Emperica Houston

Abstract

Strong evidence exists for deep Grenville (~1 Ga) oblique ramps within the Diablo Platform that have influenced structure throughout the Phanerozoic. The exposed Carrizo Mountain Group (CMG), the core of the Grenville orogen in Trans-Pecos Texas, comprises a long-lived structural high within the platform. During the 1060-980 Ma Grenville orogeny in the Trans-Pecos, the ~1.33 Ga volcano-sedimentary CMG was ductilely and brittley deformed under NW-directed, predominantly dextral transpression, ultimately being thrust over carbonates of the 1250 Ma Allamoore Fm., and depositing and folding alluvial fans comprising the Hazel Fm.

In NW exposures of the CMG, ~980 Ma structures swing from the NE strike that dominates to the SE, to the WNW trend dominating the foreland. Biotite-grade, oblique thrust-sense D1 mylonites show that this is not later open folding imposed upon D1 structure. Instead, the trend change is a part of the D1 deformation. The mylonitic foliation follows the same change in bedding, contacts, and S1 foliation, but mylonitic lineations all plunge moderately SE. In other words, transport direction is consistently to NW, but the foliation change marks a change in orientation of the flattening plane. This suggests flow over a NE striking, SE dipping slightly oblique ramp, and a WNW striking, SSW dipping oblique lateral ramp. These ramps may have been constructed entirely during the Grenville orogeny by ~980 Ma (i.e. as horses), or they may represent inversion of ~1.33 Ga faults within or bounding the CMG basin.

This change in trends occurs in regional structure from the Grenville to the Quaternary. Parallel, convergent, or tangential relations between structures show that the proposed ramps, and possible parallel ramps, exerted strong tectonic control. They explain the change from the dominant NE CMG structure to the WNW trend of the foreland fold belt, and the curve in the Grenville front. The NE CMG trend, and the brittle, Grenville-aged Mineral Creek Fault Zone, are also tangential to curved, post-Wolfcampian faults that parallel the Grenville front, or converge with those that do. (One of these faults connects with the post-Albian Victorio flexure, which parallels the fold belt.) These faults all converge upon a basin-bounding Cenozoic fault. Some of these faults may have provided the alternative inlets to the Delaware Basin that have been suggested by previous authors.

 

AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014